Scintillators have found widespread usage for the detection of neutron radiation, as emitted by many radioactive sources. Scintillation occurs due to the recombination of ionized electrons and molecular ions that are generated within a luminescent material. Two types of excited molecular states may be populated upon ionization recombination in organic scintillators, termed singlet and triplet states. Singlet excited states may freely relax to the ground state through a spin-allowed fluorescence transition that occurs on the order of nanoseconds. Triplet states, which account for 75 percent of the excited states generated by ionizing radiation interaction, may not relax freely to the ground state due to the spin-forbidden nature of this transition. Accordingly, the triplet states do not generate significant luminescence in organic scintillators due to very long luminescence lifetimes (on the order of milliseconds or longer) and low emission quantum yields. In some organic scintillating materials, delayed singlet luminescence may be observed. Delayed singlet luminescence occurs as the result of the fusion of two excited triplet excited states, which results in the production of one excited singlet and one ground singlet state. The singlet excited state rapidly decays via a delayed fluorescence mechanism, with the rate-determining step being the kinetics of triplet diffusion. Typically, only a small fraction (such as two percent) of excited electrons in triplet states may undergo this recombination and relaxation process to produce luminescence. This is due to a combination of finite triplet mobility rates and lifetimes, trapping effects, and quenching by impurities such as aromatic ketones or triplet species such as molecular oxygen. Consequently, effective materials for particle discrimination include liquid and solid organic materials that possess sufficiently large triplet mobility rates and delayed fluorescence intensities.
The prompt fluorescence response and slower delayed fluorescence response described above may be exploited in scintillator systems to discriminate between ionizing particles, including discrimination between fast neutrons and gamma ray photons. These neutral particles are converted to charged particles by interaction with the scintillation material. Elastic collisions of fast neutrons with hydrogen atoms in the scintillating material generate recoil protons, whereas interactions with gamma rays produce scattered electrons, as understood in the art. Particle discrimination is possible because the intensity of prompt scintillation is dependent upon the energy deposited per unit length in the material (dE/dX), which is lesser for electrons than protons. By contrast, the delayed scintillation response is generally less dependent upon the type of ionizing particle. This effect may be used to differentiate signals from different particle types using a technique referred to as pulse-shape discrimination (PSD).
Standard formulations for liquid scintillators comprise fluorescent solutes dissolved in aromatic organic solvents such as benzene, toluene, xylenes, and 1,2,4-trimethylbenzene. Such solvents generally possess high vapor pressures and are rated as highly flammable, flammable, or combustible. Existing solvents used in liquid scintillators possess hydrogen-to-carbon ratios of close to one, which determines the cross section for elastic proton recoil following fast neutron interaction. (Benzene has a proton density of 4.1·1022 atoms/cm3). Another property of aromatic-based liquid scintillators is a nonzero solvent absorption coefficient in the UV-Visible region, which leads to optical attenuation lengths of less than five meters at a wavelength of 420 nm.
Alternative liquid scintillator formulations have been developed to avoid these derogatory effects. In particular, mineral oil-based scintillators possess reduced volatility and flash points, but have shorter optical attenuation lengths that degrade performance in large-scale applications. The commercial liquid scintillator EJ-309 produced by Eljen Technology has been reported to have an optical attenuation length on the order of one meter, as measured at 420 nm.
Separately, liquid scintillation cocktails have been developed for the detection of tritium or charged particles in environmental test samples. Liquid scintillation counting is applied by adding a radioactive sample to a scintillation cocktail and monitoring the generated luminescence. Such materials generally employ surfactants or extracting agents to achieve phase compatibility with aqueous test samples. Alpha/beta pulse shape discrimination has been reported in several types of liquid scintillation cocktails. The method for alpha/beta discrimination is analogous to neutron/gamma PSD. Alpha/beta misclassification ratios of between 0.5 percent and 1.0 percent have been reported for discrimination of 5.5 MeV alpha particles and 710 keV beta particles in liquid scintillation cocktails. This level of discrimination efficiency is, in general, insufficient for passive detection applications such as screening at point-of-entry crossings.